Motto: Just because it’s complex, doesn’t mean we can’t make good predictions!

Tiny creatures and swampy plants, living hundreds of millions of years ago, breathed in vast quantities of carbon dioxide, storing the carbon in their bodies. Unmindful of their sacrifice, this early life performed a sort of global air-conditioning service for us. Their bodies stored enough carbon to change the atmosphere from being inhospitable to mammals like us, into one in which we thrive. Their dead bodies became the coal and oil that now power for our iPods, cars and air-conditioners.

As power plants and vehicles breathe back out the store of carbon, they set off a complicated chain of events. The climate system is one of the most complex ever studied. The path upon which the released carbon sets us may be complex, but the end-game is simple. Releasing all that carbon would undo what that ancient life did. That story begins in Climate Science For Dummies – III – The Bad Stuff.

Complex as it is, the role of carbon in our evolving climate is well understood. The question is not whether we understand the mechanisms – we do! – but how accurately we can predict the future. The climate system is ‘chaotic’, meaning there are ways it is stable, but also ways it can change rapidly into something quite different.

Predicting chaotic systems is like predicting hurricanes. You know one is coming and you know it’s soon, but you can’t say exactly when . Not being able to give a precise day and time doesn’t mean you don’t understand hurricanes, and it doesn’t mean you can’t give meaningful predictions. No-one in Florida doubts the prediction “more hurricanes are coming” or the advice “you should fortify your home”, even though no-one can say what day it will happen.Predictions about climate change are similar. We can predict what will happen, but can’t say exactly when.

People sometimes think that since we can’t predict weather two weeks from now, we can’t predict the climate in twenty years. That mistakes weather for climate. For example, we have no idea what the weather will be like on Dec. 10th, 2030 in New York City. But we do know the average temperature in December of that year will be lower than the average temperature in June. Climate predictions are not about particular events, they about trends and probabilities. We’re pretty good at those predictions.

What makes climate so complicated? Things are inter-linked: carbon effects temperature, temperature effects carbon and temperature effects temperature. This is called ‘feedback’. Positive feedback means a change causes more of the same – things speed up. Negative feedback means a change causes less of the same – a braking system. Positive feedback is like a pencil balanced on the tip – any tipping causes more tipping, it’s unstable. Negative feedback is like a marble in a bowl – any rolling causes it to roll back, it’s stable. Our climate is a mix of positive and negative feedbacks.

What worries scientists are positive feedbacks, which accelerate climate change. We can’t predict exactly when positive feedbacks will occur, but we can predict they will occur, and weknow what will happen if they do. Two basic positive feedbacks look like this:

Right now, the oceans absorb about a third of the carbon we emit. The water absorbs some, as does plankton. Oceans act as a carbon sink. When temperatures rise, they will flip into being carbon sources. Climate change will cause itself to speed up. Why? Warm air causes warmer oceans, and just like a can of pop fizzing in the sun, they will release stored carbon.

Same goes for our great rain forests. The Amazon is a giant carbon storehouse. Rising temperatures causes desertification, so the Amazon will stop absorbing carbon and will release all that it holds in a great gasp. Even our soil will flip from a sink to a source. Warmer soil means microbes get more active,releasing stored carbon. That’s already happening[i]. It is predicted that by 2040[ii], living systems will begin to release more carbon than they absorb.

There’s an elephant in the room, a bigger and badder feedback than either of these. The Arctic permafrost holds huge amounts of methane, a potent greenhouse gas. Were it released, it would effectively[iii]triple the amount of carbon in the atmosphere. It’s already happening, lakes in Russia are bubbling with the stuff.

The ice at the poles reflects sunlight and heat back into space, acting as a giant mirror. As the ice disappears, the dark water that replaces it will absorb heat. As temperatures increase enough to melt the ice, the melting ice will cause temperatures to increase. This is already happening.

There are all sorts of other feedback effects, but you get the idea. Climate science tries to predict which of these changes will occur at what temperatures and what levels of carbon. It is entirely up to us when (and if) we reach those levels. The bottom line? It’s thought that the really bad stuff – all those positive feedback mechanisms – start to really kick in around 450 ppm. 500 ppm is a ‘no-go’ zone, where all bets are off and the system might be yanked out of our control. We’re at 380 ppm now …

[iii] Methane is about twenty times as effective as carbon dioxide at trapping the sun’s heat. When I say it will effectively triple the amount of carbon I mean that the amount of methane released will have the same heating effect itself as twice the current levels of carbon dioxide.